Changed based on code review to pull #657 An error occurred

Its possible that $variables['dc_array'] is an empty array which would
generate a warning. Just cleaning up this possible case.

theme/theme.inc

@@ -646,7 +646,9 @@ function islandora_preprocess_islandora_dublin_core_description(array &$variable
     }
   }
   $variables['dc_array'] = isset($dc_object) ? $dc_object->asArray() : array();
-  $variables['dc_array']['dc:description']['value'] = nl2br(filter_xss($variables['dc_array']['dc:description']['value']));
+  if (isset($variables['dc_array']['dc:description']['value'])) {
+    $variables['dc_array']['dc:description']['value'] = nl2br(filter_xss($variables['dc_array']['dc:description']['value']));
+  }
 }
 
 /**